Although little-seen, these species are widespread and highly successful throughout the northern hemisphere.

Weasels are the most characteristic members of the zoological family Mustelidae, which itself is commonly called the "weasel family". The least weasel is the smallest member of the zoological order Carnivora.

In addition to the better known species, there are a large number of exotic and obscure weasel species such as Mustela sibirica (the kolinsky, or Siberian weasel) and Poecilogale albinucha (the north African striped weasel).

The diminutive size and elongate shape of a weasel is both a strength and a terrible curse. A weasel is capable of working its way through any passage its head will fit through, allowing it to pursue burrowing rodents into their lairs where other predators cannot go. No part of their small, elongated bodies are far from the surface however, so weasels lose body heat at a tremendous rate. The furnace-like metabolism required to offset this energy loss forces them to be hyperactive, voracious hunters. A weasel needs to hunt successfully 5-10 times each day to keep from freezing in an arctic environment.